1. Field of the Invention
The present invention relates to an optical scanning device for use in an image formation device such as a laser printer or a digital copying machine that forms an image by scanning a laser beam according to an image information and exposing the image on a photoreceptor. Specifically, it relates to an optical scanning device suitable for use in a multicolored image formation device that superposes plural images formed by plural laser beams to form one image.
2. Description of the Related Art
In the conventional image formation device using the electrographic system, it is a usual exercise to scan a light beam according to an image information by an optical scanning device and form a latent image on an electrified photoreceptor, and to transfer a developed image acquired by developing this latent image to a printing paper and form the image.
Recently, accompanied with the spread of colored documents, there has been developed a full-colored image formation device that forms the developed images of each of black (K), yellow (Y), magenta (M), cyan (C) by the electrographic system, and sequentially transfers these developed images to form a full colored image.
Specially for applications that require a high speed in image formation, a so-called tandem system full-colored image formation device is being developed, which contains plural independent image formation devices, continuously transfers the developed images formed by these devices to a single transfer medium, and forms a full-colored image in one cycle.
A conventional full-colored image formation device will now be explained with reference to FIG. 5A and FIG. 5B.
As shown in FIG. 5A, FIG. 5B, an image formation device 118K, image formation device 118Y, image formation device 118M, and image formation device 118C are provided in the order of black (B), yellow (Y), magenta (M), cyan (C), from the upstream in the paper feeding direction of a paper conveyance belt 114. To each of the image formation devices, a sub-unit constituting the electrographic process is configured around a photosensitive drum 122 as the image carrier.
In this device, first an electrification device 124 electrifies the photosensitive drum 122, and then an optical scanning device 120 scans to expose a laser beam according to the image information onto the photosensitive drum 122 to form a latent image.
Next, after a development device 126 develops the latent image, the developed image is transferred to a printing paper 112 that is conveyed at a constant speed.
This process is carried out in the order of K, Y, M, C, and a fixing device not illustrated fixes the transferred image to the paper 112, which thereafter is ejected.
The optical scanning device 120 possesses a configuration such that a laser beam source (not illustrated) emits a light beam according to image information, a rotational polygon mirror 128 deflects the light beam with a constant angular velocity, and two pieces of f.theta. lenses 130 form an image on the photosensitive drum 122 as a beam spot scanning at a constant speed.
And, in the configuration shown in FIG. 5A, FIG. 5B, the optical scanning devices 120 each have the polygon mirrors 128 individually.
And, the Japanese Published Unexamined Patent Application No. Sho 62-189420 mentions that, since the aforementioned conventional colored image formation device contains plural optical scanning devices, there are problems of high cost, size expansion of the device, disturbances of the images, and the like. And, it also mentions that the above problems can be solved by employing one motor for driving to rotate plural polygon mirrors, and by integrating the optical scanning devices into one package.
There are, however, the following problems in the optical scanning device disclosed in the Japanese Published Unexamined Patent Application No. Sho 62-189420:
(1) As shown in FIG. 1, FIG. 2, FIG. 5 in the specification of the published application, the provision of plural polygon mirrors in the device increase the cost. Further, since the polygon mirrors each have differences in the flatness of each reflecting surface and the angle of inclination of each reflecting surface in the sub-scanning direction, there occur differences in the degree of blooming for each color, the fluctuations of the vertical lines, and the condensation and rarefaction of the scanning lines (the so-called, banding); and in consequence the superposed colored image can be unacceptable. PA1 (2) As shown in the embodiment in FIG. 1 through FIG. 5 in the specification of the published application, the f.theta. lenses are laid out for the number of the beams, which increases the cost (this is because the beam needs to be transmitted through the optical axis in the general f.theta. lenses, in order to secure the optical performance). PA1 (3) In the embodiment that employs four polygon mirrors (see FIG. 1, FIG. 2 in the specification of the published application), a giant load is imposed on the motor as a rotation drive unit, and it could be the unrealistic in view of the cost and the power consumption. PA1 (4) As shown in FIG. 6, in the embodiment that employs both sides of polygon mirrors 140 at the same time and reduces the number of the polygon mirrors 140 to two, though it is realistic in practical use, since the optical components including f.theta. lenses 142, reflection mirrors 144, and the like are arrayed on both sides of the polygon mirrors 140, the entire optical scanning device becomes enormously large in the lateral direction.
Although slight miniaturization can be realized by using multiple pieces of the reflection mirrors 144, not only the cost is increased for the increased number of the reflection mirrors 144, but also the adjustment mechanism for preventing the optical path deflection or the high-accuracy mechanical dimension is inevitably required, which involves a significant cost increase.